Vapour deposition technology is commonly used in industry in order to provide substrates with layers of defined material and thickness in a fully automated process, especially in vacuum chambers. As an option for controlling the correct functioning of a depositing device, the thickness of the deposited layer can be indirectly determined by depositing a film on a separate sensor, especially simultaneously to the deposition of the layer on the substrate. The philosophy is that the measurement of the thickness of the film on the sensor can provide data for deducting the thickness of the layer on the substrate.
Recently, the manufacturing and vaporisation of substrates, e.g. substrates for organic light emitting diodes (OLEDs), requires the deposition of relatively thin layers of different materials, e.g. organic and/or metallic materials, with defined layer thicknesses within small borders resp. tolerances. The deposition of these layers is usually done by inline vacuum deposition tools which are operated for several weeks without maintenance. To achieve the required layer thickness tolerances, it is necessary to measure and control the deposition rates of the used (organic) evaporation sources very accurately. The common way to measure the deposition rate of an evaporation source is to use a so called quartz crystal oscillator. Such a rate sensing oscillator can be mounted e.g. inside a deposition beam, and the organic layer can be deposited onsite the quartz crystal in parallel to the vapour deposition on the substrate. The quartz crystal oscillator is typically calibrated to the layer thicknesses on the substrate by a tooling measurement, wherein a test layer is deposited onto a substrate. The thickness of this test layer can be measured by so called ellipsometry or so called reflectometry and set in relation to the measured rate sensing oscillator signal during the test deposition.
Thereby, the so called quartz crystal oscillators are state of the art sensors to measure deposition rates in deposition tools, but there are several disadvantages, especially for industrial applications. For example, the lifetime of the quartz is very limited and is therefore determining the maintenance period for the deposition tool. Further, the measurement accuracy of these quartz crystal oscillators is often just at the limit to control the deposition rate of the evaporation sources within the required tolerances. Further, the reliability of these quartz sensors may be not sufficient to operate an industrial mass production tool with the required uptime. Also, the sensor-reading is often and easily disturbed by external factors like cooling water, vacuum fluctuations, and vibrations and so on.
These disadvantages are known by industry. Therefore, sensor suppliers offer revolver systems with e.g. six or twelve quartzes to compensate the problems of reliability and life time. That is to say, a multitude of sensors is provided to measure the layer thickness. But this also implicates further problems like different error of measurement with respect to each of these sensors, or the requirement of calibration for a multitude of sensors. In other words, e.g. for OLED mass production where it is necessary to control the deposition rate of the organic sources within specific tolerances, state of the art sensors regularly cannot guarantee an exact deposition rate within the required tolerances, especially not with a sufficient uptime and robustness.
Since some years, some equipment suppliers promote to measure the thicknesses of the deposited layers inline onside the substrates by ellipsometry or reflectometry, and to feedback this measurement to control the deposition rates of the evaporation sources. But this measurement technique is not successfully realized until now, and it is questionable if it will be possible for all layers within the required accuracy. Mainly, the following problems are limiting this technology. The measurement of the layers of a multilayer stack (which is necessary to measure directly on the substrate) is very complicate resp. laborious, and needs to be developed on new for each stack. Further, there is a minimum layer thickness, typically 10-20 nm, for enabling a measurement with sufficient accuracy. But e.g. in an OLED stack, there are typically a lot of layers which are thinner than 10-20 nm, so that especially the dopants cannot be measured by this method.